I don't give a damn if there is never another contemporary artwork on Trafalgar Square's Fourth Plinth. I know we're all meant to be up in arms that London's mayor Boris Johnson may scrap this popular metropolitan institution - so popular, in fact, that his promise to replace the avant-garde masterpieces seen there in recent years with a statue of a war hero did him no harm in getting elected...

A committee of resistance has been formed, cookery writer Prue Leith who dreamt up the Fourth Plinth commissions is to rename herself La Passionara and lead an occupation of Trafalgar Square, the anarchist battalions are massing, the Durruti Column will guard Quinn's Plinth... No pasaran!

It's not quite the Spanish civil war, but there have been some well-meaning squeaks from supporters of the Plinth. And a well-meaning squeak is what it unfortunately always has been: a philanthropic but naive attempt to popularise modern art, concocted by people who don't appear to have any idea what modern art is. For one of the most basic - and essential - definitions of what makes a modern sculptural object different from a traditional one is this: it does not stand on a plinth.

I don't buy the idea that the Fourth Plinth is just a useful space for new art, a good way of getting its message across. It comes with a secondary meaning attached, a meaning created by the plinth itself. A plinth is an architectural setting for sculpture that distinguishes it from the surrounding world, defines it as "art", ennobles and elevates it. In the Renaissance the plinth was virtually a work of art in itself. Perhaps the most elaborate base in western art is Cellini's 16th-century plinth for his statue of Perseus in Florence - he paid almost as much attention to its extravagant reliefs as he did to the statue.

By the 19th-century the plinth was a staid and boring institution (like the Fourth Plinth now). It was a manifestly pompous way of giving weak sculpture a bit of authority. And so the modernist revolution did away with it. One of the reasons Auguste Rodin is the father of modern sculpture is that his works bypass or escape the plinth by sprawling upward and outward. The Burghers of Calais make too wide a group for a conventional plinth - they spill off it. His Gates of Hell is a teeming vertical cascade of flesh that escapes into the air. The great tradition of modernist sculpture since Rodin - from Picasso to David Smith, from Barnett Newman to Richard Serra - no more belongs on a plinth than a Jackson Pollock painting belongs in a frame.

In fact, that's a good way to express what's so silly about the Fourth Plinth - what if the Royal Academy were to have a "fourth frame" exhibition in which one contemporary artist a year got to show work in an 18th-century gilt frame? It would be camp nonsense.

Rachel Whiteread's lost work House was a powerful piece of modern art for which a plinth would have been inconceivable; yet when she was commissioned to create a work for the Fourth Plinth the result was a disappointment. Her transparent cast mirroring the plinth itself had very little of her best work's emotional resonance - and how could it? She'd been invited to make a work for a plinth. It became a work about a plinth. Other artists have been equally dazzled by what the Italian 1960s iconoclast Piero Manzoni once called the "magic base" of the plinth: Marc Quinn's work was a smug statement about putting someone disabled on a plinth; Thomas Schütte's current work there expresses nothing except his obvious boredom with the commission. Only one artist made it work. Mark Wallinger's statue of Christ exploited the plinth well, contrasting its horizontal length with one small man standing there.

With Wallinger and Whiteread now competing to create Ebbsfleet's "Angel of the South", there's no danger of public art dying out in Britain. The Fourth Plinth has popularised modern art at the expense of robbing it of what makes it modern in the first place. It has taken the excitement of British art a decade ago and co-opted it. Come on Boris, do the right thing: be a modernist hero and help the British catch up with Rodin. Free our artists from the tyranny of the Plinth.